Equine Herpesvirus-1 (EHV-1) belongs to the Alphaherpesvirinae subfamily and is an important ubiquitous enzootic equine pathogen causing epidemic abortion, perinatal mortality, respiratory disease, and occasionally neurological disease in horses, causing significant economic losses to the horse industry. EHV-1 infection elicits a local immune response at the primary site of replication, as well as systemic humoral and cellular immune responses. EHV-1 infection of naïve animals induces protective immunity against re-infection lasting 4 to 8 months after the initial acute infection (Trapp et al., (2005) J. Virol. 79: 5445-5454). EHV-1 viral glycoproteins facilitate multiple aspects of the viral lifecycle including mediation of the fusion of the viral envelope with cellular membranes, intracellular virion morphogenesis, egress, cell-to-cell spread, and virus-induced cell fusion. Thus, these proteins are major antigenic determinants for both humoral and cellular immune responses and have been utilized as subunit vaccines (Laval et al., (2015) J. Virol. doi:10.1128/JVI.01589-15; Fuentealba et al., (2014) J. Virol. Methods 202: 15-18; Fuentealba et al., (2014) J. Comp. Pathol 151: 384-393). Specifically, immunization with EHV-1 gD has been shown to generate protective immune responses in mice and horses (Fuentealba et al., (2014) J. Virol. Methods 202: 15-18, Packiarajah et al., (1998) Vet. Microbiol. 61: 261-278; Zhang et al., (1998) Virus Res. 56: 11-24; Azab & Osterrieder (2012) J. Virol. 86: 2031-2044; Weerasinghe et al., (2006) Vet. Immunol. Immunopathol. 111: 59-66; Foote et al., (2006) Vet. Immunol. Immunopathol. 111: 97-108; Foote et al., (2005) Vet. Immunol. Immunopathol. 105: 47-57; Ruitenberg et al., (2001) Virus Res. 79: 125-135). EHV-1 and HSV-1 glycoprotein gDs are required for entry into cells and cell-to-cell fusion, a function which is conserved in most, but not all alphaherpesviruses (Csellner et al., (2000) Arch. Virol. 145: 2371-2385).
Despite regular and widespread vaccination, outbreaks of EHV-1 continue to occur. Current commercial vaccines that contain inactivated virus confer only partial clinical and virological protection against EHV-1 respiratory infections, because they do not stimulate cellular immune responses, specifically cytotoxic T cells that can control cell-associated viremia and virus dissemination from the respiratory track (Kydd et al., (2006) Vet. Immunol. Immunopathol. 111: 15-30). Viral-vectored vaccines express antigens within the infected cell that can be presented via MHC class 1 (endogenous) and class II (exogenous) antigen-processing routes stimulating both humoral and cell-mediated immune responses (Yeo et al., (2013) Vet. Res. 44: 16).